Polycarbonate resins are well known, commercially available materials possessing excellent physical and chemical properties which are useful in a wide variety of applications. Such polymers or resins, since they are less dense and more breakage resistant than glass, have been especially useful as substitutes for glass as, for example, in the manufacture of tail lights and stop light lenses, protective shields for fluorescent street lights, safety shields in inspection windows, windshields, windows, and the like. However, these resins have relatively low mar and chemical solvent resistance.
In order to overcome this relatively low mar and chemical solvent resistance, polycarbonate articles have been coated with various organic and inorganic protective layers which increase the mar resistance of said polycarbonate articles. One type of inorganic protective layer is comprised of glass which has been vapor deposited onto the polycarbonate substrate. Thus, for example, French Patent Specification No. 1,520,125 and the corresponding British Specification No. 1,144,099 teach that the surfaces of polycarbonates can be improved, especially rendered more scratch resistant, by vapor depositing an SiO.sub.2 layer of at least 1.mu. thickness onto the polycarbonate. This vapor deposition is accomplished by evaporating SiO.sub.2 with an electron beam evaporator source in a high vacuum in the presence of oxygen while regularly moving the polycarbonate article to be coated in the vapor jet and/or the electron beam evaporator source in such a manner that at least 50 successive layers are evaporated onto the surface of the polycarbonate article.
However, the articles thus prepared have been found to be not entirely satisfactory since, under high stress or temperature changes, the SiO.sub.2 protective layer tends to crack and/or separate from the polycarbonate article. In order to overcome this cracking and separation of the protective silicate glass layer, various modifications of the basic vapor deposition process have been proposed. Thus, British Pat. No. 1,313,866 teaches a polycarbonate having a vapor deposited protective layer consisting of SiOx and 5 to 10% zirconium oxide. Similarly, U.S. Pat. No. 3,645,779 teaches a synthetic polymer provided with a hard, abrasion-resistant surface free of fine hairline cracks by vapor depositing under vacuum onto the surface of said polymer a B.sub.2 O.sub.3 -SiO.sub.2 glass containing less than 5 percent by weight of Na.sub.2 O. U.S. Pat. No. 3,713,869 teaches disposing an intermediate layer between the plastic substrate and the vapor deposited glass layer for the purpose of improving the adherence of said glass layer. This intermediate layer comprises a polymerization layer which is formed by subjecting low molecular organic vapors to a glow discharge operation and depositing the polymerization products on the substrate. The organic vapors are provided by organic compositions such as acetylene, xylol, and those compounds which contain Si, preferably in a SiO bond, such as silicate acid methyl or silicic acid ethylester, and low boiling siloxanes.
Another disadvantage inherent in the prior art methods of vapor depositing glass, such as the afore-described methods of using an electron beam evaporator, resides in the fact that it is rather difficult to provide a glass coating which is of uniform thickness, has good adherence to the substrate, and possess good optical properties. This is due, in part, to the fact that the electron beam acts, in effect, as a point source, i.e., ejecting the SiO.sub.x particles of glass in a very narrow beam, with the thickness of the deposited glass layer being greatest at the point of impingement and decreasing at the areas peripheral to the point of impingement. This necessitates the application of several layers of glass to form a coating approaching uniformity of thickness.
With methods wherein radiant heating is used to vaporize the glass there are encountered the problems of heat deformation of the polycarbonate substrate. This heat deformation, besides having the obvious disadvantage, especially with large panels of polycarbonate to be used as glazing materials, of deforming the polycarbonate substrate also results in high tangential pressure forces being generated upon cooling of the coated substrate which leads to cracks in the glass layer and in separation of the glass layer from the polycarbonate substrate.